1. Field of the Invention
The present invention relates to an apparatus for analyzing the electromagnetic wave radiating from a multilayer substrate, and more specifically to an electromagnetic wave analysis apparatus for obtaining the distribution of electric currents flowing through a signal layer based on the distributed constant line approximation method or the transmission line analysis method, and computing the distribution of the electric currents of the entire multilayer substrate and the electromagnetic field intensity of the radiating electromagnetic wave based on the obtained results.
2. Description of the Related Art
The social regulations relating to electric circuit devices prohibit the radiation of electromagnetic waves and noises exceeding a predetermined level according to the strict regulations of a number of countries.
To satisfy these regulations relating to electromagnetic waves, various technologies such as a shielding technology, a filtering technology, etc. are to be adopted. In order to practically adopt these technologies, it is necessary to develop a simulation technology to quantitatively compute how effective the technologies are in reducing radiating electromagnetic waves.
Based on the above described background, the Applicant et al. have disclosed the invention of a simulation technology for computing the electromagnetic field intensity generated by an electric circuit device basically using a moment method. To establish the simulation technologies, an exact model of an electric circuit device should be prepared.
The electromagnetic field intensity generated by an object can be simulated by computing an electric current flowing through each portion of an object for substitution in a well-known theoretical equation for electromagnetic wave radiation. The electric current flowing through each portion of the object can be logically obtained by solving Maxwell's equation for an electromagnetic field. However, it is hard to solve Maxwell's equation for an electromagnetic field under optional boundary conditions for an object having an optional shape.
Therefore, all analytical methods for calculating an electric current used in a current electromagnetic field intensity computation device are approximation methods, with variations in accuracy. Currently, there are three approximation methods, that is, the small loop antenna approximation method, the distributed constant line approximation method, and the moment method.
In the small loop antenna approximation method, a line connecting a wave source circuit and a load circuit is processed as a loop antenna, and the current flowing through the loop is assumed to be constant, and is obtained by a computation method for a lumped constant circuit. FIG. 1 shows the small loop antenna approximation method.
The computation by the small loop antenna approximation method is the simplest of the three methods, but is rarely used these days because of low precision when the size of a loop is not small in comparison with the wavelength of an electromagnetic wave.
On the other hand, in the distributed constant line approximation method, an electric current can be obtained by applying an equation of a distributed constant line to an object which can be approximated to a one-dimensional structure. In this method, the computation is relatively simple, and the time required for computation and the amount of computation increase almost in proportion to the number of elements to be analyzed. Therefore, such phenomena as the reflection, the resonance, etc. of a line can also be analyzed. As a result, a high-speed and high-precision analysis can be realized for an object which can be approximated to a one-dimensional structure. FIG. 2 shows the configuration according to the distributed constant line approximation method.
The computation based on the distributed constant line approximation method has the problem that a high-speed and high-precision analysis can be performed for an object which can be approximated to a one-dimensional object, but cannot be performed for an object which cannot be approximated.
On the other hand, the moment method is one of the solutions of an integral equation obtained from Maxwell's equation for electromagnetic wave motion, and can be applied to an object having an optional three-dimensional shape. Practically, an electric current is computed with an object divided into small elements.
A reference document of the moment method can be H. N. Wanq, J. H. Richmond and M. C. Gilreath: "Sinusoidal reaction formulation for radiation and scattering from conducting surface" IEEE TRANSACTIONS ANTENNAS PROPAGATION vol. AP-23 1975.
In this moment method, the structure of an electric circuit device to be simulated is designed as a mesh. The mutual impedance and the mutual admittance between elements are computed in a predetermined computing process for a target frequency, and are used for substitution in simultaneous equations together with the wave source specified in the structure information such as the obtained mutual impedance, etc. The electric current flowing through each element can be obtained by solving the simultaneous equations.
That is, when a metallic object is processed, the metallic portion is designed as a mesh to be analyzed. The mutual impedance Z.sub.ij between the mesh metallic elements is obtained, and the following simultaneous equations of the moment method established among the mutual impedance Z.sub.ij, the wave source V.sub.i, and the electric current I.sub.i flowing through the mesh metallic element, are solved to obtain the electric current I.sub.i, thereby computing the electromagnetic field intensity. EQU [Z.sub.ij ][I.sub.i ]=[V.sub.i ]
where[] indicates a matrix.
When a resistance, a capacitance, and an inductance exist in the mesh, they form part of the self-impedance elements of the mesh.
Most electric circuit devices have multilayer substrates on which a power supply layer, a ground layer, and a signal layer are mounted in a layer structure separated by layers of an isolation material, when a high-density implementation is realized.
FIG. 3A shows an example of the layered structure of a multilayer substrate. FIG. 3B shows an example of the signal layer of a multilayer substrate. As shown in FIG. 3A, nine layers are laminated into a multilayer substrate. The nine layers are, sequentially from top to bottom, a signal layer 1; a first core material 2 made of epoxy-glass, etc.; a first preimpregnated layer 3 made of an insulator for adjustment of thickness, etc.; a power supply layer 4; a second core material 5; a ground layer 6; a second preimpregnated layer 7; a third core material 8; and a second signal layer 9.
The first signal layer 1 and the second signal layer 9 implement a circuit pattern using a metal such as copper foil, etc. as shown in FIG. 3B. According to the circuit pattern, circuit parts such as chip parts are arranged to implement an electronic circuit. Between the electronic circuit and the power supply layer 4 or the ground layer 6, there is a hole called "a through-hole" through which layers are electrically connected, and the electronic circuit has a power supply and is grounded.
Thus, an electronic circuit is implemented in the signal layer of the multilayer substrate, and an electromagnetic wave having a high electromagnetic field intensity radiates from the electronic circuit.
As described above, in a device for analyzing an electromagnetic wave radiating from a multilayer substrate, a signal layer by which an electronic circuit is implemented has been considered to be a main source for radiating electromagnetic waves for use in analysis. However, it has been clear that the radiation of electromagnetic waves from a power supply layer and a ground layer cannot be ignored. Therefore, it is indispensable to analyze a radiated electromagnetic wave with the influence of a ground layer taken into account.
Although the radiation of an electromagnetic wave from a power supply layer or a ground layer cannot be ignored, there has been the problem that no appropriate models of a power supply layer and a ground layer have been developed and the radiated electromagnetic waves cannot be correctly analyzed. Furthermore, there has been the problem that no effective methods have been developed to determine how an electric current flowing through a wave source and a load flows into a power supply layer and a ground layer when a complementary metal-oxide semiconductor device (CMOS) IC driver is used, thereby preventing the distribution of electric currents radiating electromagnetic waves from being correctly obtained.